The present invention relates to the measurement of electrical signals and, in particular, to the measurement of OFDM signals.
Orthogonal frequency-division multiplexing uses many orthogonal sub-carriers (e.g., 100 to 8,000), each with a relatively low symbol rate. Each sub-carrier is modulated with a conventional modulation technique (e.g., QAM). OFDM provides very efficient use of the spectrum available.
One issue with OFDM is that the peak to average power ratio may be high. This is because at any instant, many of the sub-carriers may constructively interfere. This makes it important that the circuitry have the needed dynamic range. Often, it is the power amplifiers that are most likely to have gain compression issues (e.g., the gain falls off with input magnitude).
It can be difficult to measure gain compression in the amplifiers. A technique such as a constellation diagram may show “fuzzy” clusters, but many factors could produce a similar result besides gain compression. It is possible to apply a continuous wave signal to the amplifier and compare the input to the output, but this typically will not be representative of the performance when a complex high peak to average power ratio signal such as an OFDM signal is applied.
Similarly, a technique such as measuring the composite channel power and recording gain as power is increased with an OFDM signal could be tried. Unfortunately, because compressed time domain peaks end up being averaged with uncompressed time domain valleys, this method is of limited use.